1. Field of the Invention
This invention relates to a thermal printhead which is used to print on thermosensitive paper or to cause ink transfer from a thermal transfer ribbon or film onto printing paper for example.
2. Description of the Prior Art
As is well known, thermal printheads are widely used in facsimile machines to print transmitted information on thermosensitive paper. The thermal printhead is also used in printers of the type wherein the ink of a transfer ink ribbon or film is thermally caused to be transferred onto printing paper.
For conveniently explaining the problems to be solved by the present invention, reference is now made to FIG. 7 of the accompanying drawings which shows a typical prior art thermal printhead. A similar printhead is also disclosed in U.S. Pat. No. 4,963,886 to Fukuda et al.
As shown in FIG. 7, the prior art thermal printhead 1' mainly comprises a support plate 2', a head substrate 3', a connector board 4', and a presser member 5'.
The support plate 2' is made of a metal such aluminum. The support plate functions to dissipate the heat generated at the head substrate 3' in addition to supporting it.
The head substrate 3', which is made of an insulating material such as ceramic (e.g. alumina), is elongate and adhesively bonded to the support plate 2'. The head substrate is formed with a heating resistor line 6' extending adjacent to and along one longitudinal edge of the substrate. The substrate carries a longitudinal array of drive ICs 7' (only one shown) for selectively actuating divided dot portions of the resistor line 6'. The head substrate is further formed with comb-like connection terminals 8' (details not shown) adjacent to the other longitudinal edge of the head substrate, and a wiring conductor pattern (not shown) for connecting between the drive ICs and the connection terminals.
The connector board 4' is made of an insulating material such as a polyimide film. The connector board 4' is reinforced by a backing member 10' and has a marginal portion 4a' projecting beyond the backing member 10' to overlap the head substrate 3'. The underside of the marginal portion 4a' is formed with comb-like connection terminals 9' (details not shown) in corresponding relation to the connection terminals 8' of the head substrate.
The presser member 5', which is made of a metal such as aluminum, has an anchoring base portion 5a', an intermediate presser portion 5b', and a cover portion 5c'. The anchoring portion 5a' is fixed on the connector board 4' by means of screws 12' (only one shown). The presser portion 5b' is provided with an elastic rod 11' made of rubber for example for pressing the marginal portion 4a' of the connector board 4' into intimate contact with the head substrate 3', thereby establishing electrical connection between the two kinds of connection terminals 8', 9'. The cover portion 5c' is located above the array of drive ICs 7' for protection.
For enabling the mounting of the presser member 5', the anchoring base portion 5a' is formed with perforations 13' (only one shown) for allowing penetration of the respective screws 12'. Similarly, the connector board 4' together with its backing member 10' is also formed with perforations 14' in corresponding relation to the respective perforations 13' of the presser member 5'. Further, the support plate 2' is formed with threaded holes 14' for engagement with the respective screws 12'.
Conventionally, the backing member 10' is made of glass-fiber-reinforced epoxy resin having a glass transition point of about 120.degree.-130.degree. C. Further, the backing member is attached to the connector board 4' by a layer of thermoplastic adhesive such as acrylic adhesive with a thickness of about 50 micrometers.
According to the arrangement described above, the backing member 10' has a glass transition point of about 120.degree.-130.degree. C. However, the backing member is often subjected to a high operating temperature of about 150.degree. or more because the heat generated by the heating resistor 6' is transmitted to the backing member through the metallic support plate and because the heat of the other printer components is additionally applied to the backing member. As a result, the backing member 10' softens above its glass transition point and therefore reduces in thickness under the pressure applied by the screws 12, thereby making the screws 12 loose. Further, the thermoplastic adhesive layer between the connector board 4' and the backing member 10' also softens under a high operating temperature, additionally causing loosening of the screws.
When the screws 12' become loose, the connector board 4' together with the backing member 10' may deviate positionally relative to the head substrate 3'. As a result, the electrical contact between the connection terminals 8' of the head substrate and those of the connector board becomes improper (e.g. shorting). Further, the marginal portion 4a' of the connector board may be lifted off the head substrate, thereby resulting in complete electrical disconnection.